1. Field of the Invention
This invention relates to a control unit for a converter having a circuit arrangement for the formation of a periodic control signal with a given pulse pattern.
2. Discussion of the Prior Art
In known control methods for converters, the control signal is obtained through comparison of voltages. This requires adjusting the components used, such as amplifiers, reference voltage generators, and comparators, as well as compensation of drift errors.
Known control methods for converters can be divided into those using phase gating controls and those using pulse width controls. The control speed of a pulse-width control is greater than that of a phase gating control and its harmonic content is greatly reduced. The pulse frequency can be set high, so that the harmonic content of the current remains small, with only small inductances needed in the load circuit. The harmonic spectrum can be designed with a low cost filter in mind. This reduces both the weight and the volume of the converter and improves it efficiency. The higher the pulse frequency selected, the more accurately the desired curve can approach the output voltage. An upper pulse frequency limit is set by the switching times of the converter valves and their switching losses.
Known controls for converters operate either as phase gating controls at a constant pulse frequency and with variable "on" time, or as phase gating controls at a variable pulse frequency and with either constant "on" time or constant "off" time (Heumann, Stumpe "Thyristors", 1974, pages 167, 168) or, thirdly, by the pulse-width-modulated method (BBC Reports, 1966, pages 44 to 52). In pulse-width-modulation, the intersections of a sine wave voltage with a triangular wave voltage are the switching points of the control voltage as a pulse-width-modulated voltage having two voltage levels. The ratio of the sine wave voltage to the triangular wave voltage amplitude determines the number of switching points.
In another known method for generation of a pulse-width-modulated control voltage for converters, a sine wave voltage is compared with a triangular wave voltage composed of a main triangular voltage and a reference triangular voltage (German Offenlegungschrift No. 19 45 960).
In another known method for generation of a control signal for a converter, a sine wave voltage of constant amplitude is compared with a number of DC voltages which are symmetrical to the zero line (German Patent No. 21 12 186, U.S. Pat. No. 3,820,,003). The DC voltages are varied to control the converter output voltage amplitude. This method is particularly well suited for three-phase systems.
The methods mentioned for the generation of control signals for converters, particularly the pulse control methods each require special circuits. The ease of realization of the circuit for each control method must be considered. The wave form frequencies and the amplitudes of the voltages to be compared with one another must be adjusted exactly. The components must have no drift errors.
It is an object of this invention to provide a control unit for converters capable of generating the control signals for different converters and applications in a universal way and which allows the pulse pattern of the control signals to be determined without requiring construction of a circuit with reference voltage generators and comparators.